Add retry button to Duo Chat responses

What does this MR do and why?

• Adds agentic_manual_retry_for_duo_chat_responses (gitlab_com_derisk, default off).
• Wires @retry-message from duo-ui through DuoAgenticChatView to DuoAgenticChatStateManager, which resubmits the preceding user prompt.
• Pushes the flag via gon_helper so it's available wherever Duo Agentic Chat mounts.

This MR is the frontend wire-up only — it resubmits the preceding user prompt as a fresh chat message. The duo-workflow-service has no idea the new submission is a retry, so behavior (history, credit consumption, checkpoint handling) is identical to a normal user submission.

Follow-up work (server-aware retry)

Server-aware retry is tracked in a separate MR to keep this one small:

• !238116 — plumbs an isRetry boolean from the Vue retry handler through onSendChatPrompt / startWorkflow into the WebSocket startRequest payload sent to the duo-workflow-service.
• ai-assist!5570 — the duo-workflow-service side that consumes startRequest.isRetry to swap the previous agent attempt rather than appending a new turn, per the retry semantics in epic &21289.

The feature flag agentic_manual_retry_for_duo_chat_responses remains off until all three (this MR, !238116, ai-assist!5570) are merged.

References

• Parent epic: &21289
• Issue: #587828
• Follow-up frontend MR: !238116
• Backend (duo-workflow-service): ai-assist!5570

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:agentic_manual_retry_for_duo_chat_responses)

2. Write message to the chat, wait for response see the retry button

MR acceptance checklist

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Related to #587828